1. Field of the invention
The invention relates to hybrid components which comprise at least one first part made of composite material consisting of a fibre-reinforced organic resin matrix, and at least one second part added to it and made of metal, metal alloy or ceramic material. Examples of such components may include, inter alia, a link with an added head, a turbomachine blade with an added leading edge, and a casing with added bosses.
The invention also relates to a method of manufacturing such components.
2. Summary of the prior art
Components made of composite material comprising a mass of reinforcement fibres, for example of carbon or silicon carbide (SiC), embedded in a matrix of polymerization-hardened organic resin are often used in industry, particularly in the aeronautics industry, on account of their high strength/mass ratio compared with similar components made entirely of metal. In the components with the best performance, the reinforcing fibres are made into preforms before their impregnation with resin. A preform is an assembly of sheets of fabric which are woven from the fibres intended to form the reinforcement, the sheets of fabric being cut out and assembled together to form at least partly the shape of the component to be made, and thus take up at least part of its volume. The preform may also be made as a monofabric, i.e. a fabric constituted by several sheets of fibres woven together, such a fabric exhibiting a particularly high resistance to decohesion between the sheets of fibres. However, such components made of composite material pose difficulties resulting from the general properties of the organic resin constituting the matrix, namely low hardness and low resistance to temperature, such difficulties not occurring with metal or ceramic components.
A first problem is, therefore, to make components of composite material having a fibre-reinforced organic matrix, in which the parts which are stressed and able to assume a great variety of shapes will be made of metal, metal alloy or ceramic capable of taking the stress and formed to the required shape.
The difficulty lies in the method of bonding between the organic composite part and the metal part, when the area of contact between these two parts is subjected to concentrations of stresses:
of mechanical origin when the metal part protrudes from the composite part and is subjected to stresses tending to pull it out; PA1 of mechanical origin, again, as a consequence of different moduli of elasticity or Young's moduli; PA1 of thermal origin, as a consequence of very different thermal expansion coefficients. PA1 a) making a fibre preform to the shape of the soft part of the component together with the transition layer; PA1 b) forming the transition layer by plasma projection of a molten material onto a portion of the preform, said material being weldable with the material of the hard part; PA1 c) forming the hard part of the component, particularly in rough form, on the transition layer; PA1 d) inserting the assembly constituted by the preform, the transition layer and the hard part into a mould; and PA1 e) injecting resin into the mould so as to impregnate the preform to form the soft part, polymerizing said resin, and removing the resulting assembly from the mould. PA1 f) building up the material of the hard part using flame, electric arc or plasma projection; or PA1 g) machining the second part, and welding it to the transition layer. PA1 elimination of the risk of deformation of the component as a result of a wall of the mould pushing against the hard part, which is still not precisely shaped at this stage of the process; and, PA1 saving on resin, as the elastomer prevents the very liquid resin from moulding around the hard part. PA1 a) the metal or ceramic part is not a simple coating but a prominent part of variable shape, and is welded to the matrix of the transition layer which is interpenetrated by the fibres of the first part; PA1 b) this metal or ceramic part is obtained by building it up with material, or by welding a formed part, prior to the resin impregnation of the fibre preform. PA1 a very strong bond of the welded type between the two portions; and, PA1 perfect soundness of the material of the portion made of organic composite material.
These phenomena are worsened when the metal part is rigid, and thus thick, or when at least one dimension of the area of contact is substantial, which makes it necessary to reinforce the bond by additional mechanical means such as screwing, rivetting, seaming, etc.
A second problem is therefore to effect a very strong bond between the parts of the component made of an organic matrix composite and the parts made of metal, metal alloy or ceramic.
The low resistance of organic resin to abrasion and to impact from foreign bodies poses problems particularly in the case of aircraft propellers. These same problems also arise with the blades of turbine engines for aircraft, especially the fan blades which are situated well to the front of the engines. Indeed, as such blades rotate at speeds of up to 3000 rpm, and may have a height (from root to tip) of up to 1200 mm with a thickness below 30 mm, they are particularly exposed to abrasion by sand entering the engine, or to impact from heavy foreign bodies such as birds.
To overcome these problems, the leading edge of the blade may be covered by a metal coating, but there is then the bonding problem referred to previously. German Patent 4411670 describes a blade in which the bonding of its leading edge to the remainder of the blade is reinforced by screws and seams.
This solution requires additional manufacturing operations, and the strength it achieves remains limited since an excessive number of seams will weaken the blade by the multiplication of the holes made through it. The leading edge is therefore thin and flexible, which does not allow it to withstand properly impacts from heavy objects such as birds.
The low resistance of organic matrix composite materials to localized compressions, for example of punching type, originates from the fact that reinforcement fibres have no effect under this type of stress, whereas they are very effective with respect to tensile stresses. This problem arises in the provision of fixing points for rivets or bolts in components made of composite material, for example, air intake casings, covers or cones for aircraft engines. The problem is only partly solved by using rivets or bolts with wide heads and nuts, as such width remains limited, and substantial concentrations of stresses subsist at the fixing points, requiring a reduced tightening force and an increase of the number of fixing points.
U.S. Pat. No. 4,006,999 discloses a turbomachine blade made of a composite fibre-resin material and including a metal leading edge widely covering the convex and concave flanks of the blade. The composite part of the blade is itself composed of two parts holding grids in the vicinity of the mean plane of the blade. These grids protrude forward and are embedded in the metal constituting the leading edge, so as to strengthen the bond between the leading edge and the remainder of the composite blade. However, the effectiveness of such a solution remains limited, as this bond is effected essentially at the centre of the leading edge, the bonding of the flanks of the leading edge with the remainder of the composite blade being achieved only by adherence. Furthermore, the method of making and assembling the leading edge with the remainder of the blade is not clearly apparent.
The low temperature resistance of organic resins restricts the utilization of components made of composite materials incorporating such resins. A process is known for the thermal protection of metal parts in which plasma projection is used to produce a heat insulating ceramic shield on the surface of the parts. However, the application of this process to parts made of composite material having an organic matrix gives rise to two difficulties:
Firstly, it is not applicable to parts of substantial dimensions because of the great difference between ambient temperature and the temperature at which molten ceramic sets, and of the vitreous type of fracture of the ceramic.
Indeed, on cooling, the ceramic, because it has a greater thermal expansion coefficient, will contract more rapidly with a high risk of cracking, a high compression of the composite material part, and a substantial concentration of stresses tending to bring about detachment of the ceramic layer.
Secondly, the heat released by the plasma projection of molten ceramic onto the part made of composite material will cause considerable degradation of the resin, reducing the strength of the part. The released heat will also bring about the destruction by pyrolysis of the resin in the vicinity of the surface of the part, which reduces the strength of the bond between the ceramic layer and the composite part of the component.
A process is known for the protection of components made of a composite material consisting of reinforcement fibres and organic resin, involving plasma projection of a layer of molten metal or metal alloy onto the component. This process suffers from the drawbacks described above, and although the danger of cracking may be reduced by using a malleable material, the process is restricted to the formation of thin protective coatings on components which are not greatly stressed.